The present invention relates to a block copolymer composition, to a hot melt adhesive composition comprising this block copolymer composition and to a process for preparing such block copolymer composition.
European Patent Application No. 0,479,754 discloses a process for decolorizing vinyl aromatic-conjugated diene copolymers which are obtained by anionic polymerization using an alkali metal initiator, optionally followed by coupling of living copolymer chains by the addition of a coupling agent to the polymerization mixture. Diepoxy compounds were mentioned among many others as suitable coupling agents. As the stabilizer package there was actually used a combination of tris(nonyl-phenyl)phosphite TNPP) and octadecyl-3-(3',5'-di-tert.butyl-4'-hydroxyphenyl)propionate (IRGANOX.RTM. 1076, IRGANOX.RTM. is a trade). The decolorization process involves the addition of a monocarboxylic acid of the formula R--COOH with R representing a group containing 2 to 4 carbon atoms. It will be understood that the use of acids for obtaining color-stable polymers is undesirable from a manufacturing point of view, as the use of acids puts severe demands upon the equipment due to their corrosive nature. Also in view of safety and environmental hazards it is preferred to avoid the use of acids.
From U.S. Pat. No. 4,835,200, a color stable, heat resistant hot melt adhesive composition is known comprising a monoalkenyl arene-conjugated diene block copolymer prepared by using a bromide based coupling agent, a tackifying resin, optionally a plasticizing oil and/or a petroleum wax and a specific stabilizer composition. This stabilizer composition essentially consists of four components: (i) a thio compound being 6-(4-hydroxy-3,5-di-t-butylanilino)-2,4-bis-octylthio-1,3,5,-triazine, (ii) a phenolic antioxidant, (iii) a tris-(nonylated phenyl)phosphite and (iv) an aliphatic compatible epoxy compound. The stabilizer composition employed contains a relatively large number of components, which may be the result of the use of a bromide containing coupling agent. Apart from the relatively high costs of such extensive stabilizer composition, the use of a coupling agent containing bromide is undesirable for environmental reasons. Upon waste disposal, e.g. by incineration, of articles containing the hot melt adhesive composition disclosed in said U.S. patent, there is always the risk of releasing toxic bromine compounds into the environment. Hence, there is a need for hot melt adhesive compositions which are free of bromide residues, and of halide residues in general, and which do not require the need of extensive stabilizer compositions for obtaining an adhesive composition having the desired color stability and resistance to heat degradation.
From Japanese Patent Application No. 03/285978, an adhesive composition is known comprising a linear block copolymer of a monoalkenyl aromatic compound and a conjugated diene, a tackifier resin and optionally a softening agent. In addition hereto, small amounts of stabilizers such as antioxidants and UV stabilizers may also be present. Said block copolymer is obtained by first preparing living (i.e. lithium terminated) block copolymer chains in a hydrocarbon solvent and subsequently coupling said living block copolymer chains by means of a diglycidyloxy compound of the general formula ##STR1## wherein R inter alia may be a group of the general formula ##STR2## wherein R.sup.1 and R.sup.2 both represent hydrogen, phenyl or C.sub.1 -C.sub.20 alkyl, Ph stands for phenylene and n is an integer in the range of from 0 to 10, whereby the n=0 compound occurs in an amount of 95 % by weight or more of the total weight of the diglycidyloxy compound. Actually described stabilizers are phosphorus containing antioxidants such as TNPP and UV absorbers of the benzotriazole and benzophenone type. According to said Japanese patent application, the best results are achieved when using a mixture of two different diglycidyloxy compounds as the coupling agent. It seems, however, more easy and effective from a compounding point of view to use only one diglycidyloxy compound as the coupling agent.
It is an object of the present invention to provide block copolymer compositions which have excellent color stability, shear stability and heat stability and which, when applied in hot melt adhesive compositions, result in the adhesive composition having excellent adhesive properties. It is a further object of the present invention to provide such block copolymer compositions without the use of acids, halides or any other hazardous compounds. Finally, it is a general object of the present invention to provide a process for effectively preparing the before-mentioned excellent block copolymer compositions. This inter alia implies that it is an object of the present invention to provide a process wherein coupling efficiencies of 75% or more, preferably 80% or more, can be realized. The coupling efficiency is defined as the percentage of coupled polymer chains relative to the sum of coupled and uncoupled polymer chains.